1. Field of the Invention
This invention relates to an apparatus for coating a developer on the surface of an ordinary paper sheet or a support body of other material to produce an image forming or printing medium with a coated layer of the developer.
2. Description of the Related Art
There has thus far been known in the art an image printing apparatus using a microcapsule-coated sheet bearing microcapsules which contain a dye precursor in a sealed state and having a photo-sensitive mechanical strength, in combination with an image printing medium which is prepared by coating a layer of a developer on an ordinary paper sheet or other support material selected by the user. The image printing medium is placed on an exposed microcapsule sheet in intimate contact therewith, and then the overlapped sheets are subjected to a stage of pressing development in which microcapsules of low mechanical strength in unexposed portions of the microcapsule sheet are crushed, causing the dye precursor to exude from the microcapsules and react with the developer to produce a picture image on the support body. In order to prepare the image printing medium, there has also been known an apparatus in which a developer carrier in the form of a roller or the like carrying thereon charged particles of a developer is positioned in non-contacting face-to-face relation with the support body, and the developer particles are put into flight toward the support body by the electrostatic force produced by an electric field.
As a developer carrier for use on an apparatus of this sort, it has been the general practice to employ a roller which is constituted by a cylindrical core member of metal and a sleeve-like cover member of an insulating material such as epoxy resin or the like for wrapping the circumferential surface of the core member and provided with knurling of grooves on the outer surface thereof, or by a cylindrical core member with an electrostatically piled surface. In this instance, the particulate developer is charged by friction against the roller and retained on the roller surface by the electrostatic force. The developer layer on the carrier is regulated to a predetermined thickness by a resilient blade which is pressed against the developer carrier. An electric field is produced between the developer carrier and the support body by an electrode roller which is adapted to apply direct current of high voltage to the support body.
With the related art developer coating apparatus as described above, however, if the electric field is intensified (e.g., to 5 kv/mm in a case where the gap between the developer carrier and the support body is 600 .mu.m) for the purpose of coating the developer on the support body in an amount which is necessary to secure sufficient color development, there arises a problem that discharge marks remain on the development layer on the support body as a result of the high voltage discharge despite an increase in developer coating rate. If the field strength is lowered to a level free from such discharge marks (e.g., to about 2 kv/mm in this particular case), there will occur a material drop in developer coating rate, necessitating a slow down in the support body feed speed to a level far lower than the circumferential speed of the developer carrier and as a result requiring a considerably prolonged time for the developer coating operation.
In addition, there has been a problem that existence of charged developer of opposite polarity in the grooves knurled on the circumferential surface of the developer carrier invites drops in the developer portage by the developer carrier and accordingly in the developer coating efficiency.